Thank you for the replies.

I'm not sure I realized that "no warnings" wouldn't affect warnings emitted by XML::Simple. I think I knew this in the back of my head, but I added that at some point and it must have worked--perhaps accidentally, combined with some other change--so I never considered it.

I also don't know why the warnings re-started recently.

In any case, I'm absolutely sure that $data itself was defined.

However, after a bunch more testing, I realized that I was getting these warnings for any undefined values in the data. And then I realized that the error was that SuppressEmpty => 0 was a mistake--it's supposed to be SuppressEmpty => undef (or the empty string). When I made this change, the warnings went away, and I got my desired empty XML in the output.

I have no idea why it (seemed to) work at one point and then go back to not working. However, it's clear that the SuppressEmpty setting was the right way to fix it.

Thanks again.